The use of acoustic resonators is spreading widely in RF circuits used in the field of mobile telecommunications. Conventionally, one distinguishes between Surface Acoustic Resonator (SAW) and Bulk Acoustic Resonator (BAW). In SAWs, the acoustic resonator is located on the surface of a semiconductor product while, in BAWs, it lays inside a volume delimited between a lower electrode and a higher electrode so that the acoustic wave develops in this volume.
Because of their efficiency, acoustic resonators are frequently used in radio frequency (RF) filtering and in particular in mobile telephony.
FIG. 1 illustrates a first example of a known filtering circuit which is based on a lattice structure. The circuit comprises first and second series impedances Zs 100 and 200, and third and fourth parallel impedances Zp 300 and 400. Each series impedance Zs and parallel impedance Zp is embodied with an acoustic resonator, thereafter designated as a Tunable Resonator Component (TRC) such as described in U.S. patent application Ser. No. 11/025,599, entitled “INTEGRABLE ACOUSTIC RESONATOR AND METHOD FOR INTEGRATING SUCH RESONATOR,” filed on Dec. 29, 2004, comprising a BAW type acoustic resonator associated with two partner elements, a varactor 2 and an inductor 3.
As it is known in the art, a BAW acoustic resonator is based on a dielectric medium arranged on a reflecting element, such as a Bragg mirror. Layers having different acoustic properties and different dielectric constants are stacked on a silicon substrate. Such an acoustic element is known as a Surface Mounted Resonator (SMR). Alternatively, the resonator could be of the Film Bulk Acoustic Resonator type (FBAR), namely a resonator located above a cavity to allow the reflection of acoustic waves and to avoid damping thereof.
A BAW resonator has two very close resonant frequencies, fs (series) and fp (parallel) respectively, as illustrated in FIG. 2B. In order to embody a Tunable Resonator Component based on an acoustic BAW resonator, one associates a BAW resonator with at least two partner elements and a first inductive type partner element (represented by inductor 3 in FIG. 2A) which is variable or fixed, active or passive, which resonates with the intrinsic capacity of the resonator (depending on the electrodes) in a range of frequencies located close to the series and parallel resonant frequencies. The second partner element is generally a capacitive type element (as illustrated by varactor 2 in FIG. 2A), which is made tunable according to an electric quantity, e.g., electric voltage Vc.
In the known arrangement which is illustrated in FIG. 2A, the electric voltage Vc is controlled so as to adjust the characteristics of the two impedances Zs and Zp as illustrated in FIG. 3, by slightly shifting the resonant and anti-resonant frequencies Fs and Fp for the purpose of achieving a band-pass filter as illustrated in FIG. 3. This permits the frequencies characteristics of the two impedances Zs and Zp to be adjusted.
The following references illustrate examples of the state of the art which is known in the field of filters based on BAW resonators, without any auxiliary passive components.
“High-Q Resonators using FBAR/SAW Technology and their Applications,” M. Ueda et al., MTT-Symposium 2005;
“Design Flow and Methodology on the Design of BAW components,” E. Schmidhammer et al., MTT-Symposium 2005.
The following references illustrate examples of the state of the art which is known in the field of the design of filters comprising a tunable circuitry for adjusting the central frequency of the BAW resonator.
“SiGe:C BiCMOS WCDMA Zero-IF RF Front-End Using An Above-IC BAW Filter,” Carpentier J. F., Cathelin A., Tilhac C., et al., ISSCC 2005;
“A Tunable Bandpass BAW-Filter Architecture and its Application to WCDMA Filter,” J. F Carpentier at al., MTT-Symposium 2005;
“Nouvelle configuration de filtre RF accordable en fréquence utilisant des résonateurs BAW pour une chaîne de réception homodyne WCDMA”_S. Razafimandimby et al., 6e Colloque TAISA 2005;
“Bandpass BAW-Filter Architecture with Reduced Sensitivity to Process Variations,” C. Tilhac, S. Razafimandlmby et al., 2005 IEEJ Analog VLSI Workshop.
The following references illustrate more particularly examples of the design of filters based on BAW type resonators, with auxiliary passive components:
“Improved bulk wave resonator coupling coefficient for wide bandwidth filters,” Lakin, K. M.; Belsick, J.; McDonald, J. F.; McCanon, K. T.; Ultrasonics Symposium, 2001 IEEE.
“A Film Bulk Acoustic Resonator (FBAR) Duplexer for USPCS Handset Applications,” P. Bradley, et al., MTT-Symposium, 2001.
The latest development in the field of mobile telecommunications leads to new needs.
When considering the design of a BAW type acoustic resonator capable of operating at a reduced frequency, such as 1 Ghz for instance, one has to realize a piezzo-electric layer with an increased width, what shows to be a tricky operation with the known manufacturing techniques.
One should take note of the increasing request of customers for multiband mobile telephones. One may recall that the operating frequency of the European GSM network is based on a 850 Ghz frequency in the United States, and 900 Mhz in Europe. For the DCS European standard, the frequency of 1800 Mhz has to be considered and the PCS American standard is based on the 1900 Mhz frequency. Therefore, when one wishes to design a multiband mobile telephone that is capable of operating in different countries in the world, it becomes necessary to use, either different selective filters dedicated to specific band and arranged with multiplexors, or RF filters capable of handling different bands of frequencies.
Conventionally, a multiband mobile telephone is based on the use of different specific filtering circuits, each circuit comprising his own acoustic resonators operating at a predetermined band as well as specific receiving circuits.
This known technique has the advantage of being simple to carry out but has the drawback of requiring different arrangements of BAW resonators in a same mobile telephone.